Racingh11
Super Member
I bought this amp in rough shape as a project to match my other PM860 beater amp. I should have taken before pictures. The inside was coated in heavy dust that was also oily. And all of the black sheet metal parts had the paint pealing off. I opted to fully disassemble the amp. Ultrasonic cleaned everything and stripped the metal parts down and repainted. I installed a handle and additional rubber feet to match the other PM860 I have. Makes them real handy for mobile use.
I replaced all electrolytic capacitors and all small signal TO92 transistors throughout. I also installed one of my own input boards that makes it bridgeable and switchable low-pass filter with bass boost for a subwoofer, and a turn-on muting delay. It also got gold RCA inputs to replace the 1/4" phone plugs
After everything was assembled and settings were all adjusted, it was tested on the distortion analyzer. It seemed fine until I approached high power and distortion rose before the rated power. After connecting the oscilloscope to the high voltage transformer connection to the power board, I could see it was only charging the caps on every other phase. I traced this back to a bad solder joint at a diode used to sense that phase. This is why the previous owner installed a 15A fuse instead of the 10A stock. Because these amps have to charge at twice the amperage when it happens half as often, and they blow the fuse. It now has a maximum of .0113% THD at 205W 8 ohm. I also tried bridging to 8 ohm, tests .006% THD up to the 300W my loads could handle. IMD is even lower at .0033%.
Another thing I did on this amp was install a C13 power cord socket. This makes it much easier to work on and gives cord options. This is the second time I have installed a heavier awg power cord. Stock is only 18 awg. The last amp I tried had increased 1% THD clipping limit with a heavier cord. But I didn't test before to verify that was the reason. On this amp I was able to switch between a 10' 14 awg cord and a 6' 18 awg cord. I have 6' 14 awg ordered.
The 14 awg cord powered the 1%THD RMS clipping level all the way to 260W per channel 8 ohm, both driven at 1khz. The 18 awg cord brought it down to 220W per channel. (rated 205W) The cord alone was worth 40W per channel increase in RMS power!
I replaced all electrolytic capacitors and all small signal TO92 transistors throughout. I also installed one of my own input boards that makes it bridgeable and switchable low-pass filter with bass boost for a subwoofer, and a turn-on muting delay. It also got gold RCA inputs to replace the 1/4" phone plugs
After everything was assembled and settings were all adjusted, it was tested on the distortion analyzer. It seemed fine until I approached high power and distortion rose before the rated power. After connecting the oscilloscope to the high voltage transformer connection to the power board, I could see it was only charging the caps on every other phase. I traced this back to a bad solder joint at a diode used to sense that phase. This is why the previous owner installed a 15A fuse instead of the 10A stock. Because these amps have to charge at twice the amperage when it happens half as often, and they blow the fuse. It now has a maximum of .0113% THD at 205W 8 ohm. I also tried bridging to 8 ohm, tests .006% THD up to the 300W my loads could handle. IMD is even lower at .0033%.
Another thing I did on this amp was install a C13 power cord socket. This makes it much easier to work on and gives cord options. This is the second time I have installed a heavier awg power cord. Stock is only 18 awg. The last amp I tried had increased 1% THD clipping limit with a heavier cord. But I didn't test before to verify that was the reason. On this amp I was able to switch between a 10' 14 awg cord and a 6' 18 awg cord. I have 6' 14 awg ordered.
The 14 awg cord powered the 1%THD RMS clipping level all the way to 260W per channel 8 ohm, both driven at 1khz. The 18 awg cord brought it down to 220W per channel. (rated 205W) The cord alone was worth 40W per channel increase in RMS power!
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